At present, nonaqueous electrolyte batteries including a nonaqueous electrolyte solution such as a lithium primary battery and a lithium ion secondary battery are used in various fields. For example, the nonaqueous electrolyte batteries are used as power sources of portable devices, or find applications where the batteries are exposed to high temperatures and strong vibrations, including the use of the batteries as power sources of pressure sensors located inside tires. With the widespread use of the nonaqueous electrolyte batteries, many attempts have been made to improve their various properties.
A nonaqueous electrolyte battery that uses metallic lithium or a lithium alloy such as a lithium-aluminum alloy as a negative electrode active material, particularly a coin-type lithium primary battery, generally includes a nonaqueous electrolyte solution containing LiClO4 as an electrolyte, which has a high ionic conductivity and can provide excellent discharge characteristics. However, when the battery is stored at high temperatures, a reaction between the electrolyte solution and the electrodes can occur, leading to expansion of the battery. Therefore, in the applications where the battery is used in the high temperature environment, measures need to be taken to suppress the reaction between the electrolyte solution and the electrodes.
To deal with this issue, a sulfur compound such as propane sultone has been known as an additive that forms a coating on the surface of the positive electrode or the negative electrode so as not to react with the electrolyte solution, and thus can suppress expansion of the battery during the high temperature storage (see Patent Document 1).
However, if the electrolyte solution contains at least a certain amount of the above compound to obtain a sufficient effect of suppressing the expansion of the battery, the coating formed on the surface of the electrode interferes with the discharge reaction and increases the internal resistance of the battery. Consequently, the discharge characteristics are likely to be reduced after the high temperature storage.
On the other hand. Patent Document 2 discloses a battery that includes a nonaqueous electrolyte solution containing lithium bis(oxalato)borate [LiB(C2O4)2] and LiBF4 at a molar ratio of 2:8 to 5:5. The use of this nonaqueous electrolyte solution prevents an increase in the internal resistance due to liberation of water from a positive electrode active material into the electrolyte solution at high temperatures, or an increase in the internal pressure due to decomposition of the electrolyte solution. Thus, the battery can have excellent properties both at low temperatures and at high temperatures.
LiBF4 is an electrolyte salt having relatively high heat resistance. When the nonaqueous electrolyte solution contains LiBF4 in combination with LiB(C2O4)2, the battery can have excellent storage properties up to a temperature of about 100° C. However, in the environment at higher temperatures, even if the nonaqueous electrolyte solution contains LiB(C2O4)2, the properties of the battery are likely to be reduced by the reaction with water.
Therefore, further studies are required to prevent the degradation of the properties when the battery is stored under severe conditions at higher temperatures, e.g., in a high temperature environment of 110° C. or more.